Evidence from chlorin nitrogen isotopes for alternating nutrient regimes in the eastern Mediterranean Sea

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doi: 10.1016/j.epsl.2009.12.009
Author(s): Higgins, Meytal B.; Robinson, Rebecca S.; Carter, Susan J.; Pearson, Ann
Author Affiliation(s): Primary:
Harvard University, Department of Earth and Planetary Sciences, Cambridge, MA, United States
University of Rhode Island, United States
Volume Title: Earth and Planetary Science Letters
Source: Earth and Planetary Science Letters, 290(1-2), p.102-107. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0012-821X CODEN: EPSLA2
Note: In English. 55 refs.; illus., incl. sketch map
Summary: Nitrogen isotopes of chlorins, degradation products of chlorophyll, reflect the isotopic composition of nutrient N utilized by marine phytoplankton communities. Here we show that in sediments of the eastern Mediterranean Pleistocene and Holocene, values of δ15N for chlorins and total nitrogen vary in concert, with a consistent offset of ≈5 ppm reflecting the fractionation imparted during chlorophyll biosynthesis. Samples from the Integrated Ocean Drilling Program Sites 964 and 969 were analyzed at a sampling resolution of ≈4-10 cm, clustered around sapropel events 2, 3, 4 and 5 (≈100-170 ka). In low organic content sediments, chlorin values of ≈0 ppm coincident with total nitrogen values of ≈+5 ppm indicate that the latter reflects the original biomass and is not a consequence of diagenetic isotope enrichment. In sapropel horizons, the chlorin and total nitrogen values are 5 ppm more negative (≈-5 ppm and ≈0 ppm, respectively), resembling previously-reported, modern-day water-column particulates (≈0 ppm). We suggest that nutrient conditions in the Eastern Mediterranean correspond to three scenarios and that the similarity between sapropel and modern-day bulk δ15N is coincidental. Organic-poor marl sediments formed under oligotrophic conditions where surface productivity resulted from upwelling of Atlantic-sourced nitrate. Sapropels were characterized by enhanced diazotrophy that was likely fueled by increased riverine P fluxes to surface waters. Present-day conditions are dominated by anthropogenic N sources. These scenarios agree with a model of sapropel formation in which stratification caused by increased fresh-water inputs led to N fixation due to P:N nutrient imbalance. Enhanced production combined with stratification promoted and maintained anoxic deep waters, consequently increasing organic matter preservation. Such a model may be relevant to interpreting other episodes of intense organic matter deposition in past oceans. Abstract Copyright (2010) Elsevier, B.V.
Year of Publication: 2010
Research Program: ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 24 Surficial Geology, Quaternary Geology; Biogenic processes; C-13/C-12; Carbon; Cenozoic; Chlorins; Chlorophyll; Communities; Diagenesis; East Mediterranean; Holocene; Human activity; Isotope fractionation; Isotope ratios; Isotopes; Leg 160; Marine environment; Marine sediments; Mediterranean Ridge; Mediterranean Sea; N-15/N-14; Nitrogen; Nutrients; ODP Site 964; ODP Site 969; Ocean Drilling Program; Organic compounds; Paleo-oceanography; Paleoenvironment; Phosphorus; Phytoplankton; Pigments; Plankton; Pleistocene; Productivity; Quaternary; Sapropel; Sediments; Stable isotopes; Total organic carbon
Coordinates: N361538 N361538 E0174500 E0174500
N335024 N335024 E0245300 E0245300
Record ID: 2010045510
Copyright Information: GeoRef, Copyright 2017 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands